The size of transactions is dependent on the number of inputs used to create the transaction, and the number of outputs.
Bitcoin is Secure
In the blockchain, bitcoins are registered to bitcoin addresses. Creating a bitcoin address is nothing more than picking a random valid private key and computing the corresponding bitcoin address.
This computation can be done in a split second. But the reverse computing the private key of a given bitcoin address is mathematically unfeasible and so users can tell others and make public a bitcoin address without compromising its corresponding private key. Moreover, the number of valid private keys is so vast that it is extremely unlikely someone will compute a key-pair that is already in use and has funds. The vast number of valid private keys makes it unfeasible that brute force could be used for that.
To be able to spend the bitcoins, the owner must know the corresponding private key and digitally sign the transaction. The network verifies the signature using the public key. This was when difficulty was much lower, and is no longer feasible. Semi-log plot of relative mining difficulty [d]  Mining is a record-keeping service done through the use of computer processing power. Every 2, blocks approximately 14 days at roughly 10 min per blockthe difficulty target is adjusted based on the network’s recent performance, with the aim of keeping the average time between new blocks at ten minutes.
In this way the system automatically adapts to the total amount of mining power on the network. Mining pool Computing power is often bundled together or “pooled” to reduce variance in miner income.
Individual mining rigs often have to wait for long periods to confirm a block of transactions and receive payment. In a pool, all participating miners get paid every time a participating server solves a block. This payment depends on the amount of work an individual miner contributed to help find that block. To claim the reward, a special transaction called a coinbase is included with the processed payments.